A prodrug is an active drug chemically transformed into a per se inactive derivative which by virtue of chemical or enzymatic attack is converted to the parent drug within the body before or after reaching the site of action. The process of converting an active drug into inactive form is called drug latentiation. Prodrugs can be carrier-linked-prodrugs and bioprecursors. The carrier-linked prodrug results from a temporary linkage of the active molecule with a transport moiety. Such prodrugs are less active or inactive compared to the parent active drug. The transport moiety will be chosen for its non-toxicity and its ability to ensure the release of the active principle with efficient kinetics. Whereas the bioprecursors result from a molecular modification of the active principle itself by generation of a new molecule that is capable of being a substrate to the metabolizing enzymes releasing the active principle as a metabolite.
Prodrugs are prepared to alter the drug pharmacokinetics, improve stability and solubility, decrease toxicity, increase specificity, and increase duration of the pharmacological effect of the drug. By altering pharmacokinetics the drug bioavailability is increased by increasing absorption, distribution, biotransformation, and excretion of the drug. Limited intestinal absorption, distribution, fast metabolism, and toxicity are some of the causes of failure of drug candidates during development. Avoidance of the foreseeable or proven pharmacokinetic defects thus assumes considerable significance in drug research. Accordingly, prodrugs play a significant role in drug research as well.
In designing the prodrugs, it is important to consider the following factors: a) the linkage between the carrier and the drug is usually a covalent bond, b) the prodrug is inactive or less active than the active principle, c) the prodrug synthesis should not be expensive, d) the prodrug has to be reversible or bioreversible derivative of the drug, and e) the carrier moiety must be non-toxic and inactive when released.
Prodrugs are usually prepared by: a) formation of ester, hemiesters, carbonate esters, nitrate esters, amides, hydroxamic acids, carbamates, imines, mannich bases, and enamines of the active drug, b) functionalizing the drug with azo, glycoside, peptide, and ether functional groups, c) use of polymers, salts, complexes, phosphoramides, acetals, hemiacetals, and ketal forms of the drug. For example, see Andrejus Korolkovas's, “Essentials of Medicinal Chemistry”, pp. 97-118.
The discovery and characterization of endothelium-derived nitric oxide (NO) was the subject of the 1998 Nobel Prize in Medicine and Physiology. NO is a major signaling molecule with important biological roles. See, for example, Kerwin, Jr., J. F. et al., J. Med. Chem. 1995, 38, 4343, and Williams. R. J. P., Chem. Soc. Rev., 1996, 77. The major biological functions of NO include controlling blood pressure, smoothing muscle tone and inhibition of platelet adherence and aggregation, assisting the immune system in destroying tumor cells and intracellular pathogens and participating in neuronal synaptic transmission. See, for example, Moncada, S. et al., Pharmacol. Rev. 1991, 43, 109; Bredt, D. S. et al., Anuu. Rev. Biochem., 1994, 63, 175; Schmidt, H. H. W. et al., Cell 1994, 78, 919; Feldman, P. L. et al., Chem. and Eng. News. 1993, 71 (20th December issue), 26; and Wilsonm E. K., Chem. and Eng. News. 2004 (8th March issue), 39. Endogenously, NO is produced from arginine by the catalytic action of nitric oxide synthase. See, for example, Nathan, C. et al., Cell 1994, 78, 915, and Marietta, M. A., Cell 1994, 78, 927.
NO is a free radical as well as a scavenger of free radicals. NO reacts quickly with ubiquitously generated reactive oxygen species (ROS) such as superoxide (O2−) to generate a nefarious peroxynitrite (ONOO−) molecule, which is implicated in many human diseases such as diabetes, heart disease, Alzheimer's disease and multiple sclerosis. In this setting, NO is often viewed as pathogenic. However, the chemistry of NO can also be a significant factor in lessening the injury mediated by reactive oxygen species (ROS) and reactive nitrogen oxide species (RNOS). There is a relationship between NO and oxidation, nitrosation and nitration reactions. A number of factors determine whether NO promotes, abates or interconnects these chemistries. See, for example, Espay, et al., A chemical perspective on the interplay between NO, reactive oxygen species, and reactive nitrogen oxide species, Ann N.Y. Acad. Sci. 2002, 962, 195.
Thus, by being a free radical, along with the ability to scavenge other free radicals, NO is placed in a pivotal regulatory position. Insight into these pathophysiological processes and signaling are highly relevant to develop therapeutics.
NO deficiency has been implicated in the genesis and evolution of several disease states. In patients with cardiovascular problems, the production of superoxide is increased and level or location of NO synthesis is disrupted thereby causing cellular dysfunction as a result of vasoconstriction of blood vessels, which can lead to, if prolonged, cell damage or death. Agents that act to maintain the normal balance between NO and superoxide in vascular endothelial cells may prove particularly useful in this regard. See, for example, Stokes, K., et al., Free Radic. Bio. Med., 2002, 33, 1026-1036.
Nutritional and pharmacological therapies that enhance the bioactivity or production of NO have been shown to improve endothelium-dependent vasodilation, reduce symptoms, and slow the progression of atherosclerosis. Some of the strategies for NO modulation encompass anti-inflammatory, sexual dysfunction, and cardiovascular indications. Apart from newly developed drugs, several commonly used cardiovascular drugs exert their beneficial action, at least in part, by modulating the NO pathway. Pharmacological compounds that release NO have been useful tools for evaluating the pivotal role of NO in cardiovascular physiology and therapeutics.
NO-Donors:
There are a wide variety of structurally dissimilar organic compounds that act as NO donors and release NO in solution. Some NO donors, such as isoamyl nitrite, nitroglycerine (GTN) and sodium nitroprusside, have been used in cardiovascular medicine long before their biochemical mechanism was understood. The common mode of action for these drugs is liberation of NO, which evokes relaxation of smooth muscle through activation of guanylate cyclase with subsequent formation of cGMP. The relative importance of enzymatic versus non-enzymatic pathways for NO release, the identity of the actual NO-generating enzymes and the existence of competing metabolic events are additional important determinants of the different NO donor classes. Pharmacological compounds that release NO constitute two broad classes of compounds: those that release NO or one of its redox congeners spontaneously and those that require enzymatic metabolism to generate NO. See, for example, Ignarro, L. J. et al., Nitric oxide donors and cardiovascular agents modulating the bioactivity of nitric oxide: an overview, Circ. Res. 2002, 90, 21-28.
Nitroglycerine/glycerine trinitrate (GTN) and compounds referred to as nitrovasodilators or NO donors are frequently used in the treatment of ischemic heart disease. The common mode of action for these drugs is liberation of NO, which evokes relaxation of smooth muscle through activation of guanylate cyclase with subsequent formation of cGMP. However, early development of tolerance to nitrate therapy, particularly during acute myocardial infarction, has been the clinically significant drawback with GTN and some of the other available organic nitrates. This is a significant clinical problem and there exists a need for novel nitrate-based anti-anginal agents, which do not cause the problem of nitrate tolerance.
There are a number of new examples of organic nitrates in which an alkyl or aralkyl mononitrate is covalently linked to an existing drug molecule. Existing drugs from a large number of therapeutic areas such as anti-inflammatory, antiallergic, antibiotic, anticancer, antidiabetic, antiviral, antihypertensive, antianginal, anticonvulsant, analgesic, antiasthmatic, antidepressant, antidiarrheal, antiinfective, antimigraine, antipsychotic, antipyratic, antiulcerative, antithrombotic, etc., were made and evaluated. Some of Nicox's patents include: Synthesis and evaluation of nitrooxy derivatives of NSAIDs (WO 9412463, WO 0230867, WO 0292072, WO 0313499 and WO 0384550), aspirin (WO 9716405, WO 0044705 and WO 0104082), paracetamol (WO 0112584 and WO 0230866), antiepileptic agents (WO 0300642 and WO 0300643), COX-2 inhibitors (WO 0400781 and WO 0400300), statins (WO 04105754), ACE inhibitors (WO 04110432 and WO 04106300), and of known drugs used for the treatment of disease conditions resulting from oxidative stress and endothelial dysfunction (WO 0061537).
Most of these nitrate esters were shown to possess not only superior or equal efficacy when compared to the original drug but also exhibit much-reduced side effects. In fact, because of their superior efficacy combined with reduced toxicity, a few of such nitrate ester-containing drug conjugates are successfully passing through various stages of clinical trials. Some of Nicox's nitrooxy derivatives of drugs which are in clinical trials include: NCX 4016 (Phase II, peripheral vascular diseases), NCX 701 (Phase II, Acute pain), HCT 1026 (Phase I, Alzheimer's disease), HCT 3012 (Phase II, Osteoarthritis), NCX 285 (IND, Osteoarthritis), NCX 1022 (Phase IIa completed, Dermatitis), NCX 1020 (Phase I, Asthma/COPD), NCX 1000 (Phase I, Portal hypertension), and NCX 1510 (Phase II, Allergic rhinitis).
U.S. Pat. No. 5,767,134 and US20050002942A1 disclosed a few disulfide-containing prodrugs/folate-drug conjugates. WO 9842661, U.S. Pat. No. 5,807,847, WO 0054756 and WO 0149275 reported a few nitrooxy derivatives of organic molecules containing sulfahydryl or disulfide group which are called “SS-nitrates”. These references are incorporated herein by reference.
Representative examples from WO 9842661 have shown superior vasorelaxant activity and no tolerance was observed to the cGMP-increasing effects of those compounds under the same experimental conditions used for the induction of in vivo tolerance. WO 0149275 reports drug conjugates where an anti-inflammatory drug is covalently linked to the beta-mercapto-nitrate via thioester bond. Biotransformation pathways proposed for NO release from GTN have largely been heme-dependent or sulfahydryl-dependent. See, for examples, Thatcher, G. R. J. et al., Chem. Soc. Rev. 1998, 27, 331 and reference cited therein, and Bennett, B M. et al., Trends Pharmacol, Sci. 1994, 15, 245. These references are incorporated herein by reference.
A mutual prodrug is the association in a unique molecule of two drugs, usually synergistic, attached to each other, one drug being the carrier for the other and vice versa. The embodiments of the invention also provide mutual prodrugs, which are prodrugs of two or three therapeutic agents currently used/potential for use in combination therapy utilizing novel bio-cleavable linkers, water-soluble prodrugs of insoluble/sparingly-soluble therapeutic agents using the same linker technology and water-soluble double and triple prodrugs of sparingly-soluble therapeutic agents or any of the prodrugs linked to NO-releasing agent using the same linker technology.